For an electric power supply of electric trains or trams, modular converters comprising a plurality of converter cells configured to produce from an AC input voltage a DC output voltage which may be supplied to electrical installations on-board have recently received growing attention. Usually, the AC input voltage is supplied from a line, in particular an overhead line.
In an exemplary modular converter, each converter cell comprises a resonant DC-to-DC converter, which is connected to the line via an AC-to-DC converter. In the resonant DC-to-DC converter, a DC-to-AC converter on a line side is connected via a resonant transformer with a further AC-to-DC converter on a motor side. Both the DC-to-AC converter and the further AC-to-DC converter may be active converters comprising controllable semiconductor switches.
Frequently, a control method for the modular converter is based on hard-switching of the AC-to-DC converters' semiconductor switches and soft-switching of the DC-to-DC converters' semiconductor switches, with low current switching during a turn-off, possibly in combination with zero voltage switching (ZVS) during a turn-on of the DC-to-DC converter semiconductors.
As switching losses depend on voltage and current during switching, said switching method may result in low switching losses and thus in a high-efficiency AC-to-DC conversion at nominal power ratings.
However, when this switching method is used under light-load or no-load conditions, the switching losses of the AC-to-DC converter cells stay approximately the same, but less power is transferred, and thus efficiency decreases. In an extreme case, when no power is required to be transferred by the converter cells, switching losses in almost equal amounts would nevertheless occur.
WO 2014/037406 A1, which is hereby included by reference in its entirety, is concerned with enhancing the efficiency of a modular converter under low load or very low load conditions. Amongst other measures, this is achieved by operating the AC-to-DC converters in an intermittent mode. In said intermittent mode, in dependence on voltage and power capabilities, a first number of converter cells may be short circuited. For a second number of remaining converter cells, all AC-to-DC converters are put into an active mode when a DC-link voltage for one of the converter cells falls below an individual threshold; and are deactivated (or turned off) when a sum of all DC-link voltages of the remaining converter cells reaches a further threshold.
However, if a difference between individual thresholds and further threshold is selected sufficiently large, the DC-link voltages will oscillate between the individual threshold for the respective cell and the further threshold, which may lead to voltage oscillations and thus converter instabilities. On the other hand, if the difference between individual thresholds and further threshold is chosen relatively small, converter efficiency under low load and in particular under very low load operation remains unsatisfactory, due to the fact that the AC-to-DC converters remain in active mode.
It is an object of the invention to provide a method for controlling of a modular converter which overcomes the disadvantages as discussed above.
This object is achieved by a method in accordance with the invention.